This invention relates generally to removing a condensable vapor, such as water vapor, from a gas stream.
Freeze drying is a sublimation process that removes free water in the form of ice. Freeze drying is especially useful in the pharmaceutical industry to remove water from biological products because it preserves the integrity of the biological products. In freeze drying the water-containing product is frozen and, under vacuum with the partial pressure of water vapor reduced below the triple point of water, the frozen water sublimes and the sublimated ice is removed from the dryer.
Freeze drying has traditionally been carried out commercially using mechanical freezing systems. However, the refrigerant, such as for example a Freon, which is generally used with such mechanical devices has been deemed environmentally deleterious and is being eliminated from commercial use. Replacement refrigerants are not as thermodynamically effective making their use in the demanding application of freeze drying problematic. Moreover, replacement refrigerants for mechanical chillers are generally corrosive and toxic and require different compression ratios, making their use expensive from an operational standpoint. Moreover, an additional intermediate heat transfer fluid is needed and this has severe limitations on the temperature ranges that can be achieved.
A recent significant advancement in the field of freeze drying is the cryogenic cold shelf, disclosed and claimed in U.S. Pat. No. 5,701,745xe2x80x94Cheng et al., which uses a cryogenic fluid rather than mechanical refrigeration to carry out the freeze drying.
In practices, such as the use of cryogenic fluid to effect freeze drying, the fluid becomes loaded with water vapor which must be removed. Although mechanical refrigeration may be used for this purpose, it is disadvantageous to do so because very low temperatures are needed in order to cause the frozen water vapor to migrate out of the freeze dryer. Very low temperatures are costly to achieve using mechanical refrigeration systems. Moreover, mechanical refrigeration systems typically employ refrigeration fluids which may be damaging to the environment.
Accordingly, it is an object of this invention to provide a system for effectively removing a condensable vapor, such as water vapor, from a gas stream which can operate effectively at very low or cryogenic temperatures.
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is:
A method for removing condensable vapor from a gas stream comprising:
(A) passing liquid cryogen into a condenser comprising a plurality of spaced vertically oriented cryogen passageways, each cryogen passageway having an outer surface, and a plurality of channels, each channel having an entrance and an exit and bordered at least in part by an outer surface of a cryogen passageway;
(B) boiling the liquid cryogen within the condenser to produce cryogen vapor, and passing cryogen vapor upwardly within the cryogen passageways;
(C) passing a gas stream containing condensable vapor into the condenser, directing the gas stream to the entrances of the channels, and distributing the gas stream for substantially equal flow within the channels;
(D) passing the gas stream downwardly through the channels and condensing condensable vapor out from the gas stream onto the outer surfaces of the cryogen passageways to form a clean gas stream; and
(E) withdrawing the clean gas stream from the exits of the channels.
Another aspect of the invention is:
A cryogenic condenser comprising:
(A) a housing containing a liquid cryogen reservoir and a plurality of spaced vertically oriented cryogen passageways, each cryogen passageway in flow communication with the liquid cryogen reservoir;
(B) means for passing liquid cryogen from outside the housing into the liquid cryogen reservoir;
(C) a gas stream inlet communicating with the housing, and a plurality of vertically oriented channels within the housing, each channel having an entrance and an exit;
(D) means for directing a gas stream from the gas stream inlet to the entrances of the channels, and means for distributing the gas stream for substantially equal flow within the channels; and
(E) a gas stream outlet for passing the gas stream collecting at the exits of the channels out from the housing.